The present invention relates to a cooling system for high-power internal combustion engines and, more particularly, to an enhanced split cooling system for a diesel engine powered rail traction vehicle with a single cooling pump.
Self-propelled rail traction vehicles such as locomotives typically use diesel engines as prime movers and depend on petroleum based lubricating oil to prevent friction and wear and to provide some of the cooling in such engines. The lubricating oil, in turn, must be cooled to prevent degradation and consequential engine damage.
Because diesel engines subject lubricating oil to high temperatures in the presence of air and catalytically active metals and metallic compounds, oxidation of the oil occurs and this may lead to increased viscosity and formation of acids, carbon residue, sludge, and asphaltenes. Such changes in the oil can promote deposit formation and ring sticking, and accelerated wear and corrosion of bearing materials. Above 150.degree. F., an approximate rule of thumb is that the rate of oxidation doubles for each additional 18.degree. F. temperature rise. Therefore, higher temperature leads to shorter oil life. Additionally, oil viscosity declines with temperature, causing the lubricating film thickness in engine bearings to also decrease. If the film thickness becomes too small, aspirates on the opposing bearing surfaces will cause accelerated wear in a process known as boundary lubrication (as opposed to the desired hydrodynamic lubrication). This is especially important at high bearing loads and low shaft speeds.
As with any internal combustion engine, engine lube oil degradation necessitates oil changes at regular intervals. In order to make these oil change intervals as long as possible for economy, a controlled burn rate is established in the engine through ring design that necessitates additions of oil or "sweetening" between oil changes. In addition, rapid oxidation of the oil and/or undesirably low viscosity at the bearings is at least partially avoided during engine operation by limiting the maximum bulk temperature of the oil leaving the engine by reducing the engine power output when a maximum oil temperature set point is detected by a lube oil temperature sensor. A conservative rule of thumb for journal bearings is that the maximum film temperature should not exceed 180.degree. F. Because oil is often used for piston cooling in addition to bearing lubrication, some engines may have oil exiting the engine at temperatures approaching 250.degree. F., especially when operating in extreme conditions.
Cooling systems for internal combustion engines, such as in locomotives, are known in the art for the purpose of maintaining engine temperature and lubricating oil temperatures within desired operating parameters. In all such systems, ambient air is forced through heat exchangers and the cooling capability is constrained by the temperature of the ambient air as well as other factors. For example, U.S. Pat. No. 5,201,285 describes a controlled cooling system for a turbocharged internal combustion engine having liquid coolant to absorb heat from the engine jackets and turbochargers ("turbos"), a pump to circulate coolant through the cooling system, a fan to force air in heat exchange with a primary coolant radiator, and a secondary coolant radiator ("subcooler") in the upstream air flow of the primary radiator. The subcooler provides lower temperature coolant to remove heat from the hot, compressed engine intake air in the charge air cooler ("intercooler") which beneficially increases the charge air density and lowers the quantity of exhaust pollutants. In this system the engine lubricating ("lube") oil cooler is positioned just prior to the coolant pump in the coolant circuit, where it utilizes the entire coolant flow for the purpose of lowering the engine lube oil temperature.
U.S. Pat. No. 5,415,147 (the '147 patent) describes a temperature regulating system for a turbocharged internal combustion engine having one coolant fluid pump and one or more flow paths where coolant fluid may be directed, depending on the engine operating conditions. In one flow path, heated coolant from the engine is cooled by a primary radiator with a split outflow such that a portion may be further cooled in a subcooler that is in the upstream air flow of the primary radiator. The coolant portion flowing through the subcooler is directed either to an engine intake air intercooler or back to a reservoir. In another flow path, some heated engine coolant may be directed to the intercooler to heat the engine intake air.
The temperature regulating system of U.S. Pat. No. 5,415,147 uses three "Modes" of operation described as follows:
Mode 3: Some hot coolant outflow from the engine is used to heat the engine intake air in the intercooler. Radiators and subcoolers are drained. Mode 3 is used when the coolant is at its coldest, such as when warming the locomotive after starting a cold engine.
Mode 2: Radiators and subcoolers are used to cool some hot coolant outflow from the engine. The remainder is used to heat the engine intake air in the intercooler. Mode 2 is used when coolant temperature is high enough to warrant cooling with the radiators but is not high enough to indicate that engine intake air cooling in the intercooler is necessary.
Mode 1: Radiators and subcoolers cool all the hot coolant outflow from the engine. Coolant passing through the subcoolers is used to cool the engine intake air in the intercooler. Mode 1 is used when coolant temperatures are highest, such as when the engine is at the highest-power levels and/or when the highest ambient air temperatures are encountered.
The temperature regulating system of U.S. Pat. No. 5,415,147 uses the particular flow paths for each of the three modes described above, along with a flow control system valving composed of one valve assembly V1, (a two-position three-way "T-Port" rotary valve shafted to an external air powered actuator and an on-off butterfly valve for drainage of radiator inlet piping), and one valve assembly V2, (a two-position three-way "L-Port" valve shafted to an external air powered actuator and an on-off butterfly valve for drainage of the subcooler outlet piping). If the air ports to the two-position actuators are labeled 1 and 2, and the flow ports to the three way valves are labeled A, B, and C, Table I shows all possible combinations of V1 and V2 positions, depending on which actuator air ports are supplied with compressed air. Three of the four combinations are used for implementing Modes 1, 2 and 3 described above.
TABLE I __________________________________________________________________________ Cooling System Mode vs. V1 and V2 Position V1 V2 V1 V1 V2 V2 Flow MODE Pos Pos 3-Way BFly 3-Way B-Fly Descript __________________________________________________________________________ 3 2 1 C to B Open C to B Open Eng to W/T & I/C Rad & S/C to W/T 2 1 1 C to A Closed C to B Open Eng tRad & I/C, S/C to W/T 1 1 2 C to A Closed A to B Closed Eng to Rad, S/C to I/C X 2 2 C to B Open A to B Closed Not Used __________________________________________________________________________
In Table I, the following abbreviations are used: Eng. for engine; W/T for water tank; I/C for intercooler; Rad. for radiator; and S/C for subcooler.
In the design of this type of locomotive cooling system, if the coolant is water, the system must be able to quickly drain the radiator, subcooler, and associated piping to the reservoir by gravity to avoid coolant freezing and consequential equipment damage in cold climatic conditions. Because the locomotive may be inclined with either end higher than the other, the design must avoid pipe or radiator runs that could lead to trapped coolant which could freeze. In the cooling system of the '147 patent, the orientation of the radiator and subcooler in the coolant-to-air heat exchanger positions the inlet at the front end of the locomotive, and there will be no trapped water in this system provided the radiator and return pipes have respective tilts greater than the greatest expected rail inclination. The location and functioning of the engine lube oil cooler in the '147 patent is similar to that in U.S. Pat. No. 5,201,285, where the entire flow of coolant is passed through the oil cooler at an inlet temperature nearly the same as that of the primary radiator outflow.
In the prior art cooling system design, lube oil cooling ability is limited because the shell and tube lube oil cooler use the entire flow of coolant that passes through the coolant pump to lower the temperature of the lube oil outflow from the engine. While the majority of this coolant is cooled solely by the primary radiators, a lesser quantity, though further cooled by the subcoolers, is later reheated by use in the intercoolers prior to rejoining the flow from the primary radiators in the coolant tank. As a consequence, the coolant temperature to the lube oil cooler is approximately the same as the primary radiator outflow temperature, and this limits the temperature reduction that may be accomplished in the lube oil cooler.
In the prior art cooling system design, locating the oil cooler upstream of the water pump disadvantageously tends to increase suction at the pump inlet. To avoid cavitation and possible damage, it is necessary to maintain a certain flow area (and weight) for the water side coolant passages of the oil cooler to avoid a large coolant pressure drop in addition to the primary task of providing heat transfer from the lube oil. Additionally, in the prior art cooling system design, having the oil cooler located between the water tank and the water pump, leaves little room for the coolant and oil piping to establish long enough runs to easily withstand inevitable assembly and transient operating misalignments. The tight space envelope also impedes assembly and maintenance tasks.